The present invention pertains to the signal processing art and, more particularly, to an improved apparatus for, and a method of redundant signal selection and fault detection and isolation.
Numerous redundant signal selection systems, which include fault detection and isolation, are known to the prior art. A particular application for such systems is in the aviation art, wherein several sensors, each sensing the same aircraft parameter, provide output data signals. Processing circuitry receives the data signals, and, in accordance with predetermined logic, selects that data signal, or combination of signals which is most likely to be representative of the acutual aircraft parameter. The output from the signal selection system is then used in aircraft control circuitry, such as the autopilot or autothrottle system.
Signal selection systems commonly employ fault detection and isolation circuitry. Such circuitry monitors the imput data signals and, in accordance with a fault detection algorithm, identifies and isolates that input data signal which exhibits a fault characteristic. To further enhance the ability of the fault detection system to isolate failures, it is known to provide equalization to all of the input data signals, thereby eliminating fixed offset errors existing between channels.
U.S. Pat. No. 4,276,648 describes one such signal selection and fault detection system. Here, multiple redundant sensor signals are processed through an equalizing and selecting network which outputs the midvalue signal and eliminates all null offsets occurring among signals. Faults of both low and high frequency are detected, resulting in the isolation of the appropriate signal channel.
Where triple redundant sensors are used, the above described prior art signal selection apparatus operates fail-passive after a second detected failure. That is, upon the failure of two of three signal channels, this system causes its output to be driven smoothly to zero, regardless of the signal produced by the remaining good channel.
In certain applications, it is preferable to remain operational after two or three failures. Thus, a system which is capable of positive fault identification of two of three channels, while continuing operation with the remaining good third channel, would fulfill a need in the signal processing art.